A conventional raster scanning radar is constructed as illustrated in FIG. 16. An antenna 901 rotates at a constant speed, radiates radar search signals successively in different directions and receives echo signals. A radar transmitting and receiving unit 907 receives echo signals; an A/D converter 902 converts the echo signals to digital data; and a primary memory 903 stores the resultant converted digital data therein. The primary memory has a memory capacity to store all the echo signals based on one radar search signal corresponding to one radar sweep. A coordinates converter 904 converts the polar coordinates of information in relation to surroundings about the radar antenna to corresponding Cartesian coordinates based on bearing signals produced by the radar transmitting and receiving unit 907 and generates signals representative of the addresses of corresponding memory elements of a display memory. The echo signals read from the primary memory are written into respective memory elements of a display memory 905 which are disposed along an imaginary sweep drawn in correspondence to a direction in which echo signals come, the addresses of the memory elements are identified by output signals of the coordinates converter 904. FIG. 17 shows a relationship between imaginary sweeps traced in the display memory 905 and some memory elements of the display memory when signals read from of the primary memory are written into the memory elements of the display memory 905. Every time the pointing bearing of the antenna 901 varies, relevant contents are refreshed in the display memory 905. Referring to FIG. 17, when a sweep SW1 is traced, echo signals based on one radar search signal and read from the primary memory are successively written into memory elements M00, M01, M11, M12 . . . disposed along the sweep. When a next sweep SW2 is drawn, echo signals supplied by the primary memory are written into memory elements M00, M01, M11, M12, M22 . . . scanned by the sweep. When another sweep SW3 is traced, echo signals are written into memory elements M00, M01, M11, M12, M22 . . . disposed along the sweep. In the same way, when another sweep SW4 is run, echo signals are written into memory elements M00, M10, M11, M21, M22 . . . disposed along the sweep. When another sweep SW5 is drawn, echo signals in response to a following radar search signal and stored in the primary memory and read from the memory are successively written into memory elements M00, M10, M11, M21, M22 disposed along the sweep. When a further sweep SW6 is run, echo signals read from the primary memory are successively written into memory elements M00, M10, M11, M21 . . . disposed along the sweep. In the same way, every time the pointing direction of the radar antenna is varied, an imaginary sweep is drawn in the display memory 905, echo signals read from the primary memory are written into the display memory. An indicating device 906 comprises, for example, a raster-scanned cathode-ray tube, respective picture elements of which correspond to respective memory elements of the display memory 905 and indicates echo signals having come from a wide range of directions, read from the display memory and applied at the input terminal thereof so that surrounding conditions are displayed on the display surface thereof.
With radar apparatuses, an object existing at a far distance is displayed as a relatively big image and the same object existing at a short distance is displayed a small image and thus the same object at a short distance is displayed as narrowly with respect to an angular direction, since a radiation beam or a reception beam formed by a radar antenna has different widths depending on the distance from the antenna or is fan-shaped. Thus, echo signals generated at short distances from the antenna are harder to be recognized on an indicator. This situation also becomes worse when a function for eliminating effects by sea refections is used.
Further, with a raster scanning radar display apparatus, if the number of memory elements are increased in order to improve quality of displayed images, the dimensions of a memory element will become small and thus images are displayed small in the vicinity of the center of the display surface. This has presented difficulties in recognizing echo signals or actually causing failure in recognizing the echo signals.
An antenna having small outer dimensions for forming a radiation beam and a reception beam having a broad horizontal width has been used in order to display as big images echo signals from objects in the vicinity of the own ship on the display surface of a radar indicator. With this arrangement, angular resolution at long distances from the own ship deteriorates on the indicating surface of an indicator and antenna sensitivity will be lowered. Thus, maximum detection range of a radar of this type will be short.
With the conventional radar apparatus in the foregoing, echo signals are written a plurality of times during one rotation of the antenna into respective memory elements of the display memory corresponding to areas in the vicinity of the radar antenna on the display surface of an indicator. Only the last echo signals written into the memory elements are maintained therein and the other signals which had been previously written therein are made invalid. If the last echo signals written lastly into memory elements are zero signals, there is presented such a problem that no echo signals will be stored in the memory elements even if echo signals have been previously written thereinto.